For certain applications, porous materials (e.g. paper) must be treated so as to impart a desired property thereto. For example, if the porous material is required to resist the penetration of liquids then a suitable coating or dipping treatment of the material may be required. Such dipping or coating treatments may also be required if it is desired to increase fire resistance or enhance strength of the material. Generally however such treatments require a relatively large amount of the treatment agent which might result in undesirable modification of other properties of the material (e.g. its air permeance).
A treatment which does not involve dipping or coating, and which is effected to improve the barrier performance of a porous material is disclosed in EP-A-0,272,798. In this process, a porous material is treated by applying a small pressure differential across the material, and treating the higher pressure side with a suspension, dispersion or aerosol of the pore modifying agent. As a result, the particulate pore modifying agent passes into the larger pores and is retained there to provide zones of high surface area which restrict the passage of micro-organisms through the material, thereby improving performance.